The design of passenger transit vehicle doors is crucial for the safety of the travelling public. As is well known in the transit industry, these doors must function in a relatively hostile environment which at least include heavy usage, temperature extremes, vibration and acceleration loads. They must open and close quickly and they must meet conflicting requirements such as being securely closed when they are in the closed position and yet they must be capable of being opened manually to provide emergency egress of passengers.
One known prior art system is described in U.S. Pat. No. 5,332,279, which is entitled POWER DOOR OPERATOR FOR MULTI-PASSENGER MASS TRANSIT VEHICLES. The teachings of the referenced patent are incorporated herein by reference thereto.
The operator described in the referenced patent has an electrical motor attached to a transmission. The transmission has an output power shaft which engages drive arms connected to rods which open and close the doors. A spring loaded clutch plate includes a pair of pins engaging a collar attached to the drive arms to maintain the drive arms in engagement with the output power shaft of the transmission.
To provide emergency egress when the doors are closed, a plunger pushes the clutch plate to disengage the drive arms from the transmission output power shaft so that the doors can be opened. Difficulties have been experienced with this prior art design because the pin and collar combination tends to bind if it is not perfectly aligned. Additionally, when the doors are opened and closed, the pins slide in a groove in the collar and experience excessive wear. This is because the pins continually exert a force on the collar to bias the drive arms into engagement with the output power shaft of the transmission.